In LFG mode you can get these in the first wing, but they seem to be rarer than the other two wild pets endemic to this raid. One often spawns in the watery section of the Halls of the Chosen (where you have to jump down terraces) before the third boss, Radiance of Azshara. They seem to be rare as secondaries. Because LFG stops at the third boss you can go back and capture them after the raid is over. And if someone has already taken them, don't fret: they do respawn.